While similar solutions might exist out there, sacy tries to be special in its own way by providing one hell of a user experience (keep your assets linked as always, but just warp them with a special tag - sacy does the rest) and outsourcing the hard stuff (serving the file, client-side caching) to the tool that does it best: The webserver.
But I'd say that the ease-of use for development is what is really unique.